[swift3] using GCD's semaphore to protect critical resources accessing from multiple threads.

gcd_semaphore.swift

class MyObject {
	let semaphore = DispatchSemaphore(value: 1)

	func update1() {
		semaphore.wait()
		defer { semaphore.signal() }

		// update some critical resources
	}

	func update2() {
		semaphore.wait()
		defer { semaphore.signal() }

		// update some resources
	}
}